The present invention broadly relates to an improved electrolyte and process for electrodepositing zinc-nickel alloys, and more particularly, to an improved aqueous acidic zinc-nickel alloy electrolyte of the chloride, sulfate, and mixed chloride-sulfate type containing novel additive agents for providing improved ductility to the electrodeposit and/or improving the uniformity in the composition of the alloy electrodeposit over a broad range of current densities.
Electrolytes incorporating controlled amounts of zinc ions and nickel ions have heretofore been used or proposed for use for depositing a zinc-nickel alloy plate of a decorative or functional type on a variety of substrates such as iron and steel, for example, to provide for improved corrosion resistance, enhanced appearance and/or to build up the surface of a worn part enabling refinishing thereof to restore its original operating dimensions. Such zinc-nickel alloy electrolytes and processes are in widespread commercial use for industrial or functional plating including strip plating, conduit plating, wire plating, rod plating, tube plating, coupling plating and the like.
While substantial improvements have been made to achieve a desired grain-refinement of the alloy electrodeposit to achieve the requisite semi-bright appearance including improved adhesion, a continuing problem has been the lack of ductility of the zinc-nickel alloy deposit resulting in micro-cracking which significantly reduces the corrosion protection of the zinc-nickel alloy deposit on the substrate. A further problem associated with electrolytes heretofore known has been the tendency of the electrodeposit to vary significantly in the quantity of nickel in the zinc-nickel alloy as a result of variations in the current density at different areas of the article being plated. Such variations in the nickel content of the electrodeposit can adversely affect subsequent treatment of the electroplated article with conventional chromium containing rinse solutions for applying a chromium-containing protective coating on the electrodeposit further enhancing its corrosion resistance. It has generally been observed that when the zinc-nickel alloy contains above about 17% by weight nickel, the application of such subsequent chromium-rinse treatments in adversely affected. It has further been observed that when the nickel concentration in the electrodeposit exceeds about 25% by weight, the deposit becomes of a darkened color which detracts from the appearance of a plated article and the chromating of such dark deposits is substantially impaired resulting in reduced corrosion resistance.
The foregoing problems and disadvantages are overcome in accordance with the present invention whereby the improved electrolyte incorporating novel additive agents and the process of electrodepositing a zinc-nickel alloy employing such electrolyte produces electrodeposits which are of substantially improved ductility, provide for increases in the nickel content in the zinc-nickel alloy deposit thereby enabling the use of lower concentrations of nickel ions in the electrolyte to achieve the same nickel content thereby providing for substantial cost savings, and which provides for increased nickel deposition in the high current density areas and suppresses nickel codeposition in the low current density areas whereby an alloy deposit is obtained which is of more uniform composition over a broad range of current density areas. The process in accordance with the present invention is therefore more economical to operate, simpler to control and provides for increased uniformity in the composition of the deposit which is possessed of improved physical and chemical properties.